A ball joint is used for rotatably and swingably connecting two portions that can move relative to each other. For example, in a suspension section of a vehicle, a ball joint is used at a portion connecting a stabilizer and a shock absorber and at a portion connecting a stabilizer and a suspension arm.
The ball joint is equipped with a ball stud, a ball seat, a housing, a support bar, and a dust cover. The ball stud has a stud portion and a ball portion. The ball seat has a bearing function and the ball portion of the ball stud is press-fitted therein. The housing holds the ball seat therein, and the support bar supports the housing. The dust cover prevents entry of foreign matter, which may inhibit sliding and swinging movement of the ball portion of the ball stud in the ball seat. In this ball joint, when pullout force is applied to the ball stud, the ball portion of the ball stud is retained by a portion of a concave portion, which overhangs the ball portion, whereby the concave portion withstands the pullout load in a direction to pull out the ball portion (for example, see Japanese Patent Application Laid-Open No. 2002-89541).
In contrast, in another conventional ball joint, a ball seat made of a resin is backed up by a housing made of steel. Therefore, when pullout force is applied to the ball stud, even in a case of breakage of the ball seat, the housing retains the ball portion of the ball stud and withstands the pullout load (for example, see Japanese Patent Application Laid-Open No. 2008-57761).
In the technique for withstanding the pullout load by the ball seat, when the pullout load is applied to the ball seat, the ball seat may not withstand the pullout load and may thereby be broken or be deformed, whereby the function of the ball joint may be degraded. If the material used for forming the entirety of the ball seat is changed to a material having higher strength, the material having high strength is inevitably exposed to the surface of a portion receiving the ball portion. In this case, the material having high strength damages the surface of the ball portion and decreases the surface roughness of the ball portion or partially abrades the ball portion, whereby the function of the ball portion as a ball joint may be degraded. Accordingly, reliability of the ball joint may be insufficient. In addition, when the material used for forming the entirety of the ball seat is changed to a material having higher strength, the production cost is increased, and reduction of production cost is difficult.
In contrast, in the technique for withstanding the pullout load by the housing made of steel, the number of parts for forming the ball joint is greater than that in the technique for withstanding the pullout load by the ball seat. Accordingly, the production cost and the weight of the ball joint are increased.